(37757) Organic Chemistry 2: Chapter 18 - Aromatic Substitution Reactions (Part 1/4)

Introduction to Reactions with Aromatic Compounds

• Chapter Focus: Understanding reactions involving aromatic compounds, specifically seven main reactions.

• Objectives:

  • Explore how mono-substituted aromatic reactions proceed.

  • Extend understanding to multi-substituted benzene ring reactions.

Electrophilic Aromatic Substitution (EAS)

• EAS reaction replaces one of the aromatic protons with an electrophile without disrupting aromaticity.

1. Bromination of Benzene

• Goal: Add bromine (Br2) to the benzene ring.

• Reagents: Catalyst complex serves as an electrophilic agent.

• Mechanism Steps:

  1. Nucleophilic Attack: The benzene double bond attacks the bromine electrophile, forming a positively charged intermediate known as the sigma complex.

  2. Energy Requirement: The formation of the sigma complex leads to a loss of aromatic stability, thus requiring a considerable amount of energy.

  3. Proton Transfer: The deprotonation of the sigma complex restores aromatic stability.

• Comparison: Unlike addition reactions that destroy aromaticity, EAS preserves it.

2. Chlorination of Benzene

• Similar to bromination but uses chlorine (Cl2).

• Reagents: Specific catalysts are required for creating the electrophilic reagent.

• Mechanism:

  1. Nucleophilic attack leads to sigma complex formation.

  2. Proton transfer restores aromatic product.

3. Iodination of Benzene

• Mechanism follows the same steps as bromination and chlorination.

• Reagents: Iodine in conjunction with suitable catalysts.

4. Sulfonation of Benzene

• Goal: Add sulfur trioxide (SO3) to the benzene ring using fuming sulfuric acid (H2SO4).

• Mechanism:

  1. Nucleophilic attack by the benzene ring onto sulfuric acid forms a sigma complex.

  2. Proton transfers restore aromaticity, completing the reaction.

• Note: For reverse reactions, dilute sulfuric acid is used.

5. Nitration of Benzene

• Goal: Introduce a nitro group (NO2) to the aromatic ring.

• Mechanism:

  1. Form a nitronium ion (NO2+) from nitric acid (HNO3) and sulfuric acid (H2SO4).

  2. Proceed with nucleophilic attack and sigma complex formation, followed by proton transfer to restore aromaticity.

  • Reduction of the nitro group to an amino group via specific reagents can be done afterward.

6. Friedel-Crafts Alkylation

• Goal: Add an alkyl group to the benzene ring.

• Reagents: Aluminum trichloride (AlCl3) necessary to generate the carbocation electrophile.

• Mechanism:

  1. Alkyl halide reacts with AlCl3 to form a carbocation.

  2. Nucleophilic attack on the carbocation results in a sigma complex.

  3. Proton transfer restores aromaticity.

• Considerations:

  1. Possible carbocation rearrangement may occur.

  2. Requires secondary or tertiary alkyl halides; primary can be viable.

  3. Polyalkylation may occur where added groups activate the benzene ring.

7. Friedel-Crafts Acylation

• Goal: Add an acyl group to the benzene ring.

• Mechanism:

  1. Acyl chloride reacts with a Lewis acid to form an acylium ion.

  2. Similar nucleophilic attack and proton transfer steps as other EAS reactions.

• Outcome: Forms aryl ketones, which can be further reduced (Clemenson reduction).

• Note: Unlike alkylation, polyacylation doesn’t happen as the acyl group deactivates the ring.

Practice Problems for EAS Reactions

• Understanding Key Concepts:

  • Distinguish correct statements about EAS processes.

  • Identify electrophiles in nitration and other reactions.

  • Recognize reagents used in halogenation reactions.

• Examples:

  • Identify products formed when benzene undergoes various EAS reactions (bromination, chlorination, nitration, etc.).

  • Illustrate the use of Friedel-Crafts reactions in synthesis.

Conclusion

• Mastery of the outlined reactions is essential for success in organic synthesis.

• Continuous practice and revision of mechanisms will enhance understanding and retention.